1. Field of the Invention
The present invention generally relates to signal conditioning circuits, and more specifically to a signal conditioning circuit that merges buffering and rectification functions and provides downward expansion, variable compression and limiting capabilities.
2. Description of the Related Art
Audio systems such as public address (PA) and digital computer multi-media systems receive audio signals through microphone inputs. PA systems use power amplifiers to amplify the signals through a speaker system while multi-media computers digitize the inputs for storage, processing or playback. The audio signals are typically provided by a person speaking into the microphone.
Often the speaker is not experienced or comfortable with using a microphone, and may hold the microphone too close or too far away or may move the microphone around as he speaks. Furthermore, a person giving a presentation may be moving back-and-forth with respect to the microphone. In a PA system, this will cause the amplified signal to become very loud, and possibly distort, or fade in and out. Similarly, the computer will record a signal whose dynamic range varies due to the fluctuations in the detected audio signal.
To reduce the unnatural and annoying variances in the amplified or recorded audio signal, these systems use conditioning circuits to compress the dynamic range of the output audio signal such that faint signals are boosted and loud signals are attenuated. This improves the perceptual quality or vocal clarity of the signal. The amount of compression in the conditioning circuit is given by a compression ratio r, which is the ratio of the amplitude change of the RMS input audio signal compared to the amplitude change of the RMS output audio signal in decibels (dB). At one extreme a compression ratio of 1:1 produces no compression, while at the other extreme the RMS value of the output is held constant regardless of the input level (this is commonly referred to as an infinite ratio). When plotted in dB, a given circuit's compression curve is generally a straight line whose slope is the compression ratio. If the compression ratio can be varied, the lines for different ratios will generally intersect at a fixed reference point called the rotation point. At the rotation point, the circuit's gain is the same for all of its available compression ratios.
At an infinite compression ratio, variances in the output audio signal that would otherwise result from an improper use of a microphone are eliminated. However, the natural dynamic range of speech (the input audio signal) is also lost. In general, if the compression ratio is too large, the output signal amplitude will flatten out and very low noise signals will be amplified. If the compression ratio is too small, the microphone input problems will remain and the output may saturate or clip for large signals. The best perceptual audio quality is usually achieved between compression ratios of 2:1 and 10:1.
Conditioning circuits such as those described in product data sheets for an "AGC Microphone and Power Amplifier," model CS4611 produced by Crystal Semiconductor of Austin, Tex., December, 1992, and an "AGC Microphone and Power AMP," model MVA611 produced by Media Vision, Inc. of Fremont, Calif., September, 1992, include separate input buffer and level detection circuits, and are packaged in 24 and 28 pin packages, respectively. The input buffer presents a high impedance to the microphone and outputs a buffer signal. The level detection circuit includes both a full-wave rectifier and a root-mean-square (RMS) circuit for computing the RMS value of the input signal. Blocking capacitors are connected at the input to the level detection circuit, and between the input buffer and a voltage controlled amplifier (VCA) to prevent small DC currents in the conditioning circuitry from adversely affecting accuracy of the VCA and the level detection circuit.
An interface circuit produces a gain signal in response to the RMS input signal and a preset compression ratio. The VCA amplifies the buffer signal by an exponential function of the gain signal to produce a compressed output signal. The compression ratio can be adjusted externally to balance the competing interests of good signal quality in the desired audio range, reduced clipping and saturation for excessively large signals, and minimal amplification of noise signals. However, when the compression ratio provides enough compression to reduce the fluctuations in the audio signal without completely flattening the speaker's dynamic range, there is insufficient amplitude limiting of large signals, resulting in clipping and distortion. Furthermore, when the person is not speaking the noise level is amplified considerably.
Another type of conditioning circuit, such as described in the product data sheet for a "Record/Playback Circuit with ALC," model TDA7284 produced by SGS-Thomson Microelectronics of St. Genif Pouilly, France, May, 1991, and a "Bipolar Linear Integrated Circuit Silicon Monolithic," model TA2011S produced by Toshiba Corporation of Tokyo, Japan, September, 1991, uses feedback to control the signal compression. The conditioning circuit's compressed output signal is fed back to scale the input signal, so that an approximately constant signal value is input to the fixed gain amplifier that produces the compressed output signal. These circuits are Full Automatic Gain Control (AGC) circuits with an infinite compression ratio, and suffer from the problems mentioned above that are associated with infinite compression ratios.
Analog Devices, Inc., the assignee of the present invention, produces audio circuits AN-133, AN-116 and AN-134 that incorporate signal conditioning. The conditioning circuitry uses a buffer to communicate the input signal to a VCA and a level detection/RMS circuit, each requiring its own blocking capacitor. The RMS signal is compared to a rotation point using conventional comparator circuits. In the AN-116, the compression ratio can be externally programmed to one of six preset values.
Conventional RMS circuits, such as those disclosed by D. Sheingold "Nonlinear Circuits Handbook," Analog Devices, Inc., pp. 398-403, 1976, fall into one of two categories: explicit or implicit. Explicit detectors square the input signal, compute its mean, and then calculate the square root. Squaring the input signal reduces the dynamic range of the detector, and the large number of components needed to implement the explicit detector reduces its accuracy. Implicit detectors use negative feedback in the log domain to produce the RMS signal. By processing the signal in the log domain, the implicit detector has an improved dynamic range. However, the feedback topology limits the practical bandwidth of the detector.
VCAs that are presently available for compression circuits, such as the that described in a product data sheet for a "Voltage-Controlled Amplifier/OVCE," model SSM-2018 produced by Analog Devices, Inc., 1981, use a negative feedback architecture around a gain core stage to produce a differential output current and a differential feedback current. A transresistance stage comprising a pair of resistors responds to a differential input voltage and the differential feedback current to produce a differential voltage. A one pole transconductance stage uses a capacitor to limit the VCA's high frequency gain to stabilize the feedback loop and supply a differential drive current to the gain core stage. The VCA also includes a second gain core stage and a differential pair of transistors for adjusting the effective capacitance of the transconductance stage to maintain a constant loop gain. The second gain core stage and its control circuitry use a large number of components to maintain the loop gain, which increases cost and reduces accuracy.